Resonant operation is useful and desirable for power devices having workpieces with reciprocating motion, due to the inherent efficiency of such devices when they are operating at or near resonance. In such a case, the driven assembly/system portion of the device which includes the workpiece is driven at or near its resonant frequency. This requires matching substantially the drive frequency of the drive system portion of the device to the resonant frequency of the driven system portion of the device. The workpiece can include, besides toothbrushes, shavers, sanders, power knives and other reciprocating tools.
In those products which are mass-produced, the tolerances of the resonant system (the driven system) must be carefully controlled. Tuning of the devices during assembly, or by using an adaptive drive system, is necessary but is typically tedious and expensive, increasing costs and raising quality control issues. Accordingly, while resonant operation is often an advantage, it is typically designed out of such devices to reduce manufacturing costs. Non-resonant systems, on the other hand, usually have problems of efficiency at relatively high operating speeds, as well as high noise levels, due to the normal operation of the drive train portion of such systems.
It is thus desirable to have a power device with a reciprocating workpiece which includes a drive system/driven system combination which operates at resonance but without the strict operating tolerances for the driven system otherwise required.